Pseudoephedrine
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 Pseudoephedrine | |
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(+)-(1S,2S)-2-methylamino-1-phenylpropan-1-ol | |
| Empirical formula | C10H15NO |
| Molecular weight | 165.2 |
| Bioavailability (Oral) | unknown |
| Metabolism | hepatic |
| Elimination half-life | 9-16 hours |
| Excretion | urinary |
| Pregnancy category | B2 (Australia) |
| Legal status | Schedule 3 (Australia) |
| Routes of administration | oral |
Pseudoephedrine is a sympathomimetic amine commonly used as a decongestant. The salts pseudoephedrine hydrochloride and pseudoephedrine sulfate are found in many over-the-counter preparations either as single-ingredient preparations, or more commonly in combination with antihistamines and/or paracetamol/ibuprofen.
This agent is often referred to by consumers as Sudafed, which is the trademark for a common brand of pseudoephedrine hydrochloride.
Unlike antihistamines, which modify the systemic histamine-mediated allergic response, pseudoephedrine only serves to relieve nasal congestion commonly associated with colds or allergies. The advantage of oral pseudoephedrine over topical nasal preparations, such as oxymetazoline, is that it does not cause rebound congestion (rhinitis medicamentosa).
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Nomenclature
Pseudoephedrine is the International Nonproprietary Name (INN) of the (1S,2S)- diastereomer of ephedrine (which has 1R,2S- configuration). Equivalent names are (+)-pseudoephedrine and D-pseudoephedrine. (Reynolds, 1989)
(-)-Pseudoephedrine or L-pseudoephedrine then designates the enantiomer of pseudoephedrine.
Chemistry
As with other phenylethylamines, it is also very chemically similar to amphetamines (like methamphetamine and methcathinone), although amphetamines are more potent and have additional biological effects which are absent from pseudoephedrine.
Mode of action
Pseudoephedrine is a sympathomimetic amine - that is, its principal mechanism of action relies on its indirect action on the adrenergic receptor system. Whilst it may have weak agonist activity at α- and β-adrenergic receptors, the principal mechanism is to displace noradrenaline from storage vesicles in presynaptic neurons. The displaced noradrenaline is released into the neuronal synapse where it is free to activate the aforementioned postsynaptic adrenergic receptors.
The vasoconstriction that pseudoephedrine produces is believed to be principally an α-adrenergic receptor response. Whilst all sympathomimetic amines, to some extent, have decongestant action; pseudoephedrine shows greater selectivity for the nasal mucosa and a lower affinity for central nervous system (CNS) adrenergic-receptors than other sympathomimetic amines. (+)-(1S,2S)-pseudoephedrine shows far lower CNS activity than other Ephedra alkaloids, ephedrine, and (-)-(1R,2R)-pseudoephedrine.
Clinical use
By acting on α-adrenergic receptors in the mucosa of the respiratory tract, pseudoephedrine produces vasoconstriction, which shrinks swollen nasal mucous membranes; reduces tissue hyperaemia, oedema, and nasal congestion. Other beneficial effects may include increasing the drainage of sinus secretions, and opening of obstructed Eustachian tubes.
Pseudoephedrine is thus indicated for:
- nasal congestion,
- sinus congestion,
- Eustachian tube congestion. (Bicopoulos, 2002)
Pseudoephedrine is also indicated for vasomotor rhinitis, and as an adjunct to other agents in the optimum treatment of allergic rhinitis, croup, sinusitis, otitis media, and tracheobronchitis. (Bicopoulos, 2002)
Pseudoephedrine is also used as first-line therapy of priapism. Erection is largely a parasympathetic response, so the sympathetic action of pseudoephedrine may serve to relieve this condition.
Adverse effects
Pseudoephedrine has been associated with the following adverse drug reactions (ADRs).
Common ADRs include: nervousness, restlessness, insomnia. (Bicopoulos, 2002)
Infrequent ADRs include: difficult/painful urination, dizziness/lightheadedness, heart palpitations, headache, increased sweating, nausea/vomiting, trembling, troubled breathing, unusual paleness, weakness. (Bicopoulos, 2002)
Misuse and illicit use
There have also been reports of off-label uses of pseudoephedrine for its stimulant properties. Some patients, long-distance truck drivers for example, have reportedly used pseudoephedrine as a stimulant to increase their state of alertness/awakedness. It is doubtful that pseudoephedrine would be of significant benefit, except in sensitive-individuals, due to its minimal effect in the central nervous system (see Mode of Action above). Nevertheless, such misuse of pseudoephedrine has been associated with stimulant dependence.
The similarity in chemical structure to the amphetamines has made pseudoephedrine a sought-after chemical precursor in the illicit manufacture of methamphetamine and methcathinone. U.S. federal law prohibits buying cold preparations containing pseudoephedrine in quantities greater than 3 packages in any 24-hour period. Individual states also have varying laws on the matter. Various other legislatures around the world similarly restrict the sale of pseudoephedrine-containing products. Internationally, pseudoephedrine is listed as a Table I precursor under the United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances[1] (http://www.incb.org/pdf/e/list/red.pdf).
Most pseudoephedrine tablets are covered with a red material. Some clandestine manufacturers take a large quantity of generic pseudoephedrine (e.g. Suphedrine), put it in a sieve with shaved ice, and swirl it around until the red material is rubbed off, leaving only the white pseudoephedrine-containing filler.
References
- Bicopoulos, D (Ed.) (2002). AusDI. Drug Information for the Healthcare Professional (2 ed.). Castle Hill: Pharmaceutical Care Information Services
- Reynolds, JEF (Ed.) (1989). Martindale. The Extra Pharmacopoeia (29 ed.). London: The Pharmaceutical Press. ISBN 0-85369-210-6